The goal of the proposed research is to obtain basic information on the mechanisms of delivery of vitamin A to the mammary gland during lactation, and on cellular retinoid binding proteins that may facilitate intracellular transport and metabolism of retinol in the breast. Two major specific aims are proposed in which the lactating rat will serve as our model. In Aim 1, we will determine the relative roles of chylomicron remnants and serum retinol-binding protein (RBP) and in delivering vitamin A to the mammary gland during lactation. We will test the hypothesis that uptake of retinol from RBP is quite constant over a wide range of dietary vitamin A, whereas uptake from the chylomicron remnant is variable and directly related to dietary consumption of vitamin A. To test this model, we will conduct a series of experiments in which lactating rats will receive an intravenous test dose of chylomicrons that have been labeled in vivo with 3H-retinyl esters. Remnant metabolism and uptake into mammary tissue will be measured as a function of time after administration, the dose of total chylomicron lipid, and the amount of chylomicron vitamin A. Experiments will also be conducted to determine whether local lipolysis is necessary for uptake of remnant retinyl esters by the breast, and whether retinyl esters are hydrolyzed and reesterified prior to secretion in milk. Complementary experiments on the uptake of 3H-retinol bound to serum RBP will also be conducted. In Aim 2, we will initiate studies on the cellular form of retinol-binding protein (CRBP) in the lactating mammary gland. Using ELISA and immunocytochemical techniques, we will determine whether CRBP in this tissue is identical to that of other organs such as liver, the subcellular distribution of CRBP in tissue fractions, and the cellular distribution of CRBP in the lactating mammary gland. We will also investigate whether CRBP participates in the metabolism (esterification) of retinol in the breast, and whether the amount of CRBP or its distribution varies with vitamin A nutritional status, or with the developmental state of the mammary gland before and during the lactation period. Information to be developed in each of these specific aims will provide insight into the mechanisms by which retinol is metabolized and transported across the breast during lactation.